Recently UWB receives attention as new communication means. In UWB, large-capacity data transfer can be realized within a short range of about 10 m by the use of a wide frequency band.
Recently a study on an ultra-wide-band filter that can be used for UWB is actively made. For example, there has been reported that a wide-band characteristic of a passband width exceeding 100% in terms of fractional band width (band width/center frequency) is obtained with a bandpass filter in which a principle of a directional coupler is applied (for example, see Non-patent Document 1).
On the other hand, a bandpass filter in which a plurality of quarter-wave stripline resonators are provided in parallel while mutually coupled is well known as a filter frequently used conventionally (for example, see Japanese Patent Publication Laid-Open No. 2004-180032).